Reappraisal of the traditional Wenckebach phenomenon with a modified ladder diagram.

Understanding of the traditional Wenckebach phenomenon is enhanced by using a modified ladder diagram where AV conduction in any cycle is represented by a slanted line in the AV bar together with similar AV conduction lines of all the preceding cycles. The diagram facilitates calculation of the duration of RR intervals (equal to the basic PP or sinus interval minus the PR or AV increment applied to this particular cycle) and the duration of the pause (equal to 2 × PP or sinus interval minus the sum of all the increments applied to the AV delay). The modified Wenckebach diagram should help students understand the mysterious clustering of QRS complexes or "paradoxical" increase of the ventricular rate that occurs during a Wenckebach sequence.

A reappraisal of pacemaker timing cycles pertaining to automatic mode switching.

Automatic mode switching algorithms of dual chamber pacemakers require fundamental changes in the operation of pacemaker timing cycles to optimize detection of supraventricular tachyarrhythmias. The timing cycles related to mode switching are basically independent of the algorithm design. Blanking periods (when the sensing amplifier is temporarily disabled) should be optimized to a relatively small fraction of the pacing cycle to enhance atrial sensing and prevent far-field sensing.